2003 Ford Explorer blows cold air when I turn on heat, how do you fix the problem?

2003 Ford Explorer blows cold air when I turn on heat, how do you fix the problem?

Added (1). Thermostat has already been replaced and flushed with new coolant.

The first thing to do is to find if it is a electrical problem with the blend door motor or control panel or the heater core its self. Get back when you know.

If you have a new thermostat the likely culprit would be a clogged or failed heater core.